A. Technical Field
The present invention relates generally to the design of bulk acoustic wave (hereinafter, “BAW”) filters, and more particularly, to the implementation of “blocker inductors” within a BAW resonator filter to improve bandwidth and suppress above band signals with the filter.
B. Background of the Invention
Resonator technology is known within the art and has been previously implemented within electrical circuit designs. Resonant elements have been used within integrated circuits such as voltage controlled oscillators, reference oscillators and filters. Examples of these resonant elements include crystal resonators, BAW resonators and surface acoustic wave (hereinafter, “SAW”) resonators. BAW and SAW resonators may be used in certain passive high-performance filter designs and implemented in various topologies such as lattice and ladder configurations. One skilled in the art will recognize the advantages of employing resonator elements within these high-frequency filters.
BAW resonators are considered to provide relatively good power handling and exhibit low frequency drift with temperature. A BAW resonator comprises a piezoelectric material that is placed between two electrodes. A BAW resonator piezoelectric layer may be made of various types of material known within the art. A BAW resonator may be manufactured using various techniques and structures including thin film bulk acoustic resonators (“FBARs”) and solidly mounted resonators (“SMR”).
The BAW resonator structure is deformed when an electric field is applied across the electrodes of the piezoelectric resonator. At certain frequencies, the BAW resonator may function as an electrical short, or a path with relatively low impedance, while at other frequencies the BAW resonator may exhibit very high impedance or function as a parallel plate capacitor.
The resonant behavior of a BAW resonator is a result of the conversion of electrical energy to acoustic energy, and the propagation of the acoustic energy across the piezoelectric material within the BAW. For example, at frequencies significantly below the resonant frequency, the efficiency of this conversion is low because the wavelength of the acoustic wave is larger than the thickness of the piezoelectric material, which results in propagation of this acoustic energy being substantially prevented. Comparatively, at frequencies significantly higher than the resonator frequency, there is very little excitation of acoustic waves in the piezoelectric layer resulting in these higher frequencies being largely blocked by the resonator.
A BAW resonator has a resonant frequency which is typically related to the thickness of the stack of piezoelectric material, associated metal end plates, shunt loading and if needed oxide layer. At frequencies at and around the resonant frequency, the piezoelectric layer is able to provide effective electro-acoustic conversion and efficiently propagate the acoustic energy through the resonator. These resonant frequencies help to define the bandwidth of a BAW resonator bandpass filter.
FIG. 1 illustrates an exemplary bandpass filter in which a plurality of BAW resonators in a ladder configuration is implemented. As shown, a first series BAW resonator 110, a second series BAW resonator 120, and a third series BAW resonator 130 are coupled in series between an input 105 and an output 135. A first shunt BAW resonator 140 and first helper inductor (L1) 170 are coupled between the first BAW resonator 110 and the second BAW resonator 120, and ground 180. A second shunt BAW resonator 150 and second helper inductor (L2) 175 are coupled between the second BAW resonator 120 and the third BAW resonator 130, and ground 180.
The first helper inductor (L1) 170 and the second helper inductor (L2) 175 are used to enhance the bandwidth of the filter and improve out-of-band rejection. This out-of-band rejection is improved by utilizing the parasitic capacitance (“Cp”) of the corresponding BAW resonator above its parallel resonant frequency (“Fp”) and may constitute a third shorting or series resonance above the passband defined at:F=1/(2Π*SQRT(Cp*Lhelper))
Comparatively, a third resonant frequency may also be established below the passband using the helper inductors defined at:F=1/(2Π*SQRT((Cp+Cm)*Lhelper)))where “Cm” is the piezoelectric equivalent capacitance of the resonator.
These “third” resonant frequencies add an additional frequency rejection notch or trap that improves the rejection of corresponding out-of-band frequencies within the BAW resonator filter.
The BAW resonator functions by receiving a signal at the input 105, and depending on its frequency, a path through the resonators is defined. If the signal frequency falls within the bandpass, then the signal is transmitted through the first, second and third series BAW resonators 110, 120, and 130 to the output 135 of the filter. At the frequency edges of this bandpass, notches are defined in which frequencies are strongly filtered. At the lower stop band, the shunt resonators short out the signal path, while at the upper stop band, the series resonators are open and therefore block the signal path. Frequencies beyond these edges (i.e., frequencies outside of the passband) are attenuated and filtered by the resonator configuration at varying levels. The depth, slope and width of these notches and attenuation of out-of-band frequencies, depend on the electrical characteristics of the resonators and the helper inductors implemented within the filter design.
The performance requirements of BAW resonator filters are becoming increasingly demanding as these types of filters are applied to different technologies and markets. The width of the filter's bandpass and the rejection characteristics of frequencies outside of this bandpass are oftentimes critical in meeting the performance requirements of a BAW resonator filter specification.